STOKING AND INCUBATION OF CULTURES 85 



This, when raised to a certain temperature, ensures a fairly 



constant distribution of the heat round the chamber. The 



latter is also furnished with double doors, the inner being 



usually of glass. Heat is supplied from a 



burner fixed below. These burners vary & 



much in design. Sometimes a mechanism 



devised in Koch's laboratory is affixed, which 



automatically turns off the gas if the light 



be accidentally extinguished. Between the 



tap supplying the gas, and the burner, is ' 



interposed a gas regulator. Such regulators 



vary in design, but, for ordinary chambers 



which require to be kept at a constant 



temperature, Reichert's is as good and 



simple as any, and is not expensive. It is 



shown in Fig. 34. 



The gas enters at a and from b passes to the 

 burner. When the mercury in / expands to cut 

 off the gas at c sufficient passes by the bye-pass e 

 to keep the flame alight. There is an improved 

 form with a large bulb filled with xylol attached 

 at /. Changes in the bulk of the xylol are com- 

 municated to the mercury. This instrument is very 

 delicate and will be found to work well. 



/ 



The varieties of incubators are, as we „ „ „ . , 

 , . , m , - , ,, Fig. 34.— Eeichert s 



have said, numerous. We have found those gas regu i a t or . 



of Hearson of London extremely good, and 



they are fitted with a good regulator. It is preferable in using 



an incubator to connect the regulator with the gas supply and 



with the Bunsen by flexible metal-tubing. It is necessary to 



see that there is not too much evaporation from the surface of 



cultures placed within incubators, otherwise they may quickly 



dry up. It is thus advisable to raise the amount of water 



vapour in the interior by having in the bottom of the incubator 



a flat dish full of water from which evaporation may take place. 



With tubes which will require to be long in the incubator, the 



plugs should be pushed a little way into the tube and a 



few drops of melted paraffin dropped on the top of the wool, 



or the plugs should be covered either by indiarubber caps 



or by pieces of sheet rubber tied over them. These caps 



should be previously sterilised in 1-1000 corrosive];' sublimate 



and then dried. Before they are placed on the tube the 



cotton-wool plug ought to be well singed in a flame. -•■,--" Cool " 



incubators are often used for incubating gelatin at"" 21° to 



